Run-of-river hydropower generation has been known to humankind since the age of antiquity, and is still in common use in the modern era. The term “run-of-river hydropower” refers to hydropower installations used for extracting hydropower with minimal interference with the water flow, typically used on flat-flowing rivers. The simplest method of run-of-river hydropower generation is based on immersing a turbine runner into the flow and converting the kinetic energy of the flow into hydropower. The turbine runners range from the simple waterwheel of ancient times to those found in the sophisticated hydroelectric turbines of the present day. Run-of-river hydropower is known to be environmentally benign and cost-effective. As more than 75% of the surface of the earth is flat, the potential of hydropower generation out of flat-flowing rivers is significant.
Flat-flowing rivers are characterized by lesser slope along the riverbed. Hence, they provide very limited “head” for energy-extraction (“head” is a measure of hydrostatic pressure in hydraulic systems, measured as the elevation of the upper surface of the liquid fed into the installation above the discharge surface level). The currently recognized upper operational efficiency limit of modern turbine technologies is known to be less than 60% of the kinetic energy of the flow running through the turbine. That upper operational efficiency limit was established in 1919 by the German physicist Alfred Betz. It is founded on both theoretical physics and practical considerations. Betz' Law—named after its author—is a theoretical physical law, supported by experimental results, that sets the upper limit of extracting kinetic energy from a flow at 0.593 (59.3%) of the total kinetic energy going through the device extracting the energy. Betz' law is currently considered a benchmark against which new flow-hydropower turbine designs are evaluated. Increasing the efficiency of the turbine in extracting the energy of the flow is one of the main objectives of turbine design.